The collapse of the Western Antarctica ice sheet is already under way and is unstoppable, two separate teams of scientists said on Monday.
The glaciers' retreat is being driven by climate change and is already causing sea-level rise at a much faster rate than scientists had anticipated.
The loss of the entire western Antarctica ice sheet could eventually cause up to 4 metres (13ft) of sea-level rise, devastating low-lying and coastal areas around the world. But the researchers said that even though such a rise could not be stopped, it is still several centuries off, and potentially up to 1,000 years away.
The two studies, by Nasa and the University of Washington, looked at the ice sheets of western Antarctica over different periods of time.
The Nasa researchers focused on melting over the last 20 years, while the scientists at the University of Washington used computer modelling to look into the future of the western Antarctic ice sheet.
But both studies came to broadly similar conclusions – that the thinning and melting of the Antarctic ice sheet has begun and cannot be halted, even with drastic action to cut the greenhouse gas emissions that cause climate change.
They also suggest that recent accumulation of ice in Antarctica was temporary.
“A large sector of the western Antarctic ice sheet has gone into a state of irreversible retreat. It has passed the point of no return,” Eric Rignot, a glaciologist at Nasa and the University of California, Irvine, told a conference call. “This retreat will have major consequences for sea level rise worldwide.”
The two studies between them suggest sea-level rise will be far greater than envisaged by the United Nations’ IPCC report earlier this year. The IPCC forecast on sea-level rise did not factor in the melting of the western Antarctica ice sheet.
The Nasa study, to be published in Geophysical Research Letters, studied the retreat of six glaciers in western Antarctica that are already the major drivers of global sea-level rise.
One of those glaciers, Pine Island, retreated 31km at its centre from 1992-2011. Rignot said all six glaciers together contained enough ice to add an additional 1.2m (4ft) to sea levels around the world.
In the University of Washington study, which will be published in the journal Science, researchers used detailed topography maps, airborne radar and computer modelling to reach greater certainty about the projected timeline of the ice sheet collapse.
The study honed in on the Thwaites glacier – a broad glacier that is part of the Amundsen Sea. Scientists have known for years that the Thwaites glacier is the soft underbelly of the Antarctic ice sheet, and first found that it was unstable decades ago.
The University of Washington researchers said that the fast-moving Thwaites glacier could be lost in a matter of centuries. The loss of that glacier alone would raise global sea level by nearly 2ft.
Thwaites also acts as a dam that holds back the rest of the ice sheet. Once Thwaites goes, researchers said, the remaining ice in the sheet could cause another 10 to 13ft (3-4m) of global sea-level rise.Satellite view of Antarctica with the Thwaites glacier marked in red. Photograph: UIG/Getty Images
“The thinning we are seeing is not just some temporary trend. It is really the beginning of a larger scale collapse that is likely to play out over a two to 10-century range,” Ian Joughin, a University of Washington glaciologist, told The Guardian.
He said the retreat would begin slowly, resulting in sea-level rise of less than 1mm a year for a couple of hundred years. But “then boom, it just starts to really go,” Joughin said.
Even under the worst-case scenario currently envisaged, the collapse of the entire ice sheet is about 200 years off – and the collapse could be as far away as 1,000 years, depending on future warming.
But collapse is inevitable, the scientists said. Joughin put the most likely timeframe at between 200 and 500 years.
The two teams of scientists used airborne radar and satellites to map the layers of ice down to the sea bed, and to study the rate of glacier movement. The Nasa team also drew on observations stretching back 40 years.This film charts the rescue of passengers stranded on the MV Akademik Shokalskiy, which became trapped in heavy ice off the coast of East Antarctica in December 2013
Even so, Rignot said he was taken aback at how fast change was occurring.
“This system, whether Greenland or Antarctica, is changing on a faster time scale than we anticipated. We are discovering that every day,” Rignot said.
Scientists are also finding that the causes of the ice loss are highly complex – and that it is not just due to warmer temperatures causing surface melting of the ice.
Both papers said the contact between the glaciers and the relatively warmer water at the ocean depths was the main driver of the slow-motion collapse.
Rignot said that even drastic action to cut greenhouse gas emissions that cause climate change could not prevent the collapse.
“We feel this is at the point where even if the ocean is not warming up, is not providing additional ocean heat, the system is in a sort of chain reaction that is unstoppable,” he told reporters on a conference call.
The only thing that could hold the glaciers back would be a large hill or big mountain that could block the retreat, Rignot said. But there is none, he said, “So we think it is not going to be stoppable.”